1. Field of the Disclosure
The present disclosure relates to micro-fluid ejection devices, such as inkjet printheads. More particularly, although not exclusively, the disclosure relates to a method of encapsulating an inkjet printhead heater chip for ion beam cross-section polishing. Also disclosed is a method of preparing a cross-section sample of the inkjet printhead heater chip.
2. Description of the Related Art
Thermal inkjet printhead heater chip, also a micro-electro-mechanical system (MEMS) device, is the heart of thermal inkjet technology. Thermal Inkjet technology is a complicated field which requires a delicate, balance of cross disciplines including mechanical engineering, electrical engineering, materials science, chemistry, fluid dynamics, aerodynamics, thermal dynamic and color science.
Thermal inkjet printhead heater chips have a unique construction, consisting of ink vias, hundreds of thin film heaters, ink chambers/nozzle holes, power transistors, logic circuits and contact pads for electrical interconnections. The ink chambers and nozzles are formed by using a thick polymer film having a thickness of 20 to 30 microns. Below the nozzles and ink chambers are thin film heaters, which are about 3 microns thick. During the inkjet heater chip development and ink development, it is important to evaluate the heater chip thin film construction, ink chamber/nozzle construction, heater surface degradation, and ink/heater surface interaction after printing. This evaluation is commonly done by preparing cross-sectional samples of the inkjet heater chip. However, the unique construction of the inkjet heater chip presents a great challenge to preparing quality cross-section samples using typical mechanical polishing and focused ion beam (hereinafter ‘FIB’) techniques.
Mechanical polishing has been the typical way of preparing cross-section samples for inspection of the thin film profile of inkjet printhead heater chips. Mechanical polishing is abrasive in nature. In protecting the heater chip samples for mechanical polishing, two part epoxy adhesive and a cover glass are traditionally used to encapsulate the heater chips. The two-part epoxy adhesive is an adhesive composition including a first part having an epoxy resin, and a second part having a curing agent. Due to the unique nozzle/chamber structure on top of the thin film heaters, the two-part epoxy adhesive tends to form bubbles at the corners and has poor adhesion to heater surface. The glass/epoxy encapsulation gives enough protection to the polymer nozzle holes and polymer ink chambers, but not enough protection for the thin film stack, as a result, the heater this film stack often delaminates from substrate due to the physical stress during grinding. Such damage deteriorates the interlayer stack of the inkjet heater chip samples and makes it very difficult to accurately measure the thin film thickness. In addition, mechanical polishing also results to rough surfaces which make it impossible to get any identification and measurement of ink build-up or damage on fired heater surface.
In FIB cross-sectioning, inkjet printhead heater chips require a cut that has to be at least 30 microns deep in order to reach the heater thin film stack, so the process is very time consuming. Another problem associated with FIB cross-sectioning of inkjet heaters is the beam damage to heater surface through the open nozzle hole without protective coating. However, depositing platinum as a protective coating is impossible due to the 20 μm to 30 μm deep nozzle. If a polymer protective coating is applied on nozzles, it will be difficult to find the center of the nozzle in FIB using scanning electron microscope, and the process requires even longer cut time.
Because of the problems associated with mechanical polishing and FIB cross-sectioning for sample preparation, ion beam cross-section polishing has been identified as an alternative method for preparing inkjet heater chip cross-section samples. An ion beam cross-section polisher is commercially available from JEOL Ltd, Tokyo, Japan. With ion beam cross-section polishing, the ion beam slowly cuts through the chip, thus a fine cross-section of chip is exposed. Due to the larger ion beam (˜150 microns in diameter) of the ion beam cross-section polisher, the sample preparation time is reduced, and the cross-section area is dramatically increased. However, since ion beam polishing uses an ion beam like the FIB, the ion beam still damages the heater surface through the nozzle holes if a protective coating is not applied.
Several materials, such as thin glass cover combined with the two part epoxy adhesive have been tested as coating layers to protect the heater surface from the ion beam damage during the ion beam cross-section polishing. However, due to low ion beam cutting rate of glass, it takes a long time just to cut through the glass cover, so the cost of sample preparation is high when accounting for machine time and mask consumption. The glass cover may be removed, followed by lapping the epoxy adhesive to thin down the protective coating. However, this lapping procedure requires extensive experience and is very time consuming. The glass cover may be replaced with thin silicon to cover the heater chip. The ion beam etches the silicon cover faster than the glass cover, but the non-transparency of the silicon cover to the built-in light microscope in ion beam cross-section polisher makes it difficult to polish a specific area of interest. Other materials such as super glue and a two part epoxy without any cover glass have been tried. However super glue has its limitations, including low viscosity, slow curing, and relatively poor wettability to the heater chip. These limitations result to the super glue not easily staying on the sample surface to provide sufficient protection. A limitation of the two part epoxy is being too viscous to flow into the ink channel to cover the heater surface, resulting in air bubbles and uneven coating.
In preparing the inkjet printhead heater chip samples for ion beam cross-section polishing, it is desired to provide the heater chip with a coating layer that would protect it from damage when exposed to the ion beam. The coating layer needs to have good adhesion to the nozzle plate, the ink flow chamber and the heater surface. Moreover, the coating layer needs to have thin and uniform thickness, be free from voids, and have good mechanical strength and high temperature properties when placed under the ion beam.
Thus, it is desired that the formulation for the coating layer have a rheology that allows it to flow into channels, wet and cover the heater surface, and quick enhancement of the viscosity, thereby allowing the coating layer to stay on top of the heater surface. In addition the formulation for the coating layer has to have a polymer thermal reflow property which allows a process known in the art as “self-healing.” Self-healing avoids voids between the coating layer and the heater chip. Furthermore, the coating layer has to have a final cure that provides sufficient mechanical strength.